Investigations will focus on the following aspects of host-parasite relationships in EB virus infections: 1) clinical, epidemiologic, and virologic aspects of primary infections in two age groups, children and young adults; 2) identification of specific cells in the parotid gland and/or in the oropharynx in which EBV replication occurs; 3) elucidation of the possible role of EBV in the pathogenesis of RANA positive cases of rheumatoid arthritis; 4) defining more precisely the cells that express EBNA, and factors which may modify this; 5) analysis of virus functions using viral mutants to determine the manner in which lymphocytes regulate EB virus expression.